I dig into the sources and I think you correctly pinpoint the bug. It
seems we have a mismatch between the local and remote sizes in the
inter-communicator allgather in the 1.2 series (which explain the
message truncation error when the local and remote groups have a
different number of processes). Attached to this email you can find a
patch that [hopefully] solve this problem. If you can please test it
and let me know if this solve your problem.